1. Field of the Invention
The present invention relates to a machine tool for moving a spindle on which a cutting tool is mounted relatively to a work and feeding the cutting tool to the work in a predetermined direction of feeding, to subject the work to working.
2. Description of the Prior Art
In order to shorten working time, a machine tool comprising a spindle unit of a type capable of rotating the cutting tool at a high speed and supported by a magnetic bearing has been conventionally widely employed.
The magnetic bearing supporting type spindle unit comprises
i) a main body, PA1 ii) a spindle serving as a rotating shaft passed through the main body for transmitting torque to a cutting tool, PA1 iii) an electromagnet for radially supporting the spindle with a clearance therebetween by its magnetic force, PA1 iv) a position sensor for sensing the position in the radial direction of the spindle, and PA1 v) magnetic force controlling means for finding the deviation between the position in the radial direction of the spindle obtained on the basis of an output signal from the position sensor and the target position in the radial direction of the spindle with respect to the main body (the amount of the shift in the position in the radial direction from the target position of the spindle), and controlling the magnetic force of the electromagnet so as to cancel the deviation and maintain the spindle at the target position.
In order to further shorten the working time, in the machine tool comprising the magnetic bearing supporting type spindle unit, one employing a servo motor having the capacity to move the spindle unit relatively to the work and feed the cutting tool to the work at a high speed (for example, 10 to 15 m/min) in a predetermined direction of feeding at the time of working has been spreading in recent years. FIGS. 10A to 10F illustrate an example of working using a machine tool having the above-mentioned high-speed feeding function. In FIGS. 10A to 10F, a work 2 is moved, to feed a cutting tool 1 composed of an end mill to the work 2 at a predetermined high speed along a direction of feeding M, cause the cutting tool 1 to cut through the work 2 crossing the direction of feeding M from its one end surface 2a to the other end surface 2b, and process a surface to be cut 2c of the work 2. L denotes a working line to be traced by the cutting tool 1, and F denotes an external force received by the cutting tool 1.
The cutting tool 1 is subjected to a change in the external force F rapidly increased from zero when it cuts into the one end surface 2a as shown in FIGS. 10A and 10B, while being subjected to a change in the external force F rapidly reduced to zero when it cuts through the other end surface 2b as shown in FIGS. 10E and 10F. The larger the feed speed is, the higher the rate of change in the external force F is.
When an attempt to increase the feed speed to perform working at a high speed is made, therefore, the position of the cutting tool 1 is rapidly shifted in the direction in which the amount of cutting into the surface to be cut 2c is decreased (downward in the drawings) with respect to the main body of the spindle unit in the initial stages of working (see FIGS. 10A and 10B), while being rapidly shifted in the direction in which the amount of cutting is increased (upward in the drawings) in the final stages of working (see FIGS. 10D, 10E and 10F). In the intermediate stages of working shown in FIG. 10C, the external force F becomes approximately constant, so that the cutting tool 1 is almost moved on the working line.
On the other hand, in the spindle unit, a magnetic force exerted to reduce the shift in the position to zero is produced. However, the production of the magnetic force cannot follow the above-mentioned rapid shift in the position without the delay. Therefore, in the initial stages and the final stages of working, the amount of the shift in the position from the working line L exceeds an allowable range Q as shown in FIG. 11. Therefore, insufficient cutting 2d exceeding the allowable level occurs in the initial stages of working, and excessive cutting 2e exceeding the allowable level occurs in the final stages of working, as shown in FIG. 10F.